The present invention relates to navigation systems, and more particularly, to methods of improving the performance of navigational systems by utilizing one or more external sources of information, for example a global positioning (GP) system (also referred to herein as GPS).
Typical prior art inertial guidance (IG) systems can calculate position with a high degree of accuracy. To attain such accuracy, these IG systems require precise gyroscopes that are extremely costly, but are characterized by a low measurement error, typically on the order of 100 degrees per hour. By contrast, many commercial applications use a lower grade of gyroscope, typically a micro-machine gyroscope, which is relatively inexpensive. These micro-machine gyroscopes include a relatively large measurement error, typically ranging from approximately one degree to ten degrees per second. Such low-grade gyroscopes are most suitable for gross movement detection (e.g., detection of automobile roll-over, and air bag deployment systems) rather than fine movement detection required by an IG system.
Gyroscope measurement error can generally be divided into the categories of bias error and scaling error. All gyroscopes have a certain degree of measurement error that is present upon initialization, referred to as bias error. The instrument using the gyroscope can apply some measure of compensation for this error, but the effectiveness of such compensation is limited because the actual degree of initialization error may be different for each individual gyroscope. In addition, each time an individual gyroscope is turned on, the amount of bias error may be different. For instance, one initialization might result in an error of one degree per second and the next initialization might result in an error of two degrees per second.
The second category of error, referred to herein as scaling error, that accumulates over the angle through which a gyroscope is being rotated. Scale factor error is essentially an xe2x80x9cinput to outputxe2x80x9d error, i.e., the difference between the actual angle of rotation the gyroscope experiences and the angle of rotation indicated at the gyroscope output. A gyroscope indicating that it had turned ninety degrees when it had, in fact, turned ninety-two degrees, is an example of scaling error. The amount of scaling error may be affected by various environmental factors, so that a fixed compensation value will not be sufficient to produce completely accurate data.
GPS navigation systems are widely used and are rapidly being incorporated into many newly manufactured commercial vehicles. Such vehicles often operate in city environments, however, resulting in substantial blackout periods while in so-called xe2x80x9curban icanyons,xe2x80x9d i.e., while between tall buildings that obscure the line-of-sight to the GPS satellites. A collocated IG system can provide continuous navigational information during these blackout periods, but the high cost of the precise gyroscopes required by typical prior art IG systems virtually precludes their use in a commercial vehicle. Hence, a general need exists for a method of improving the accuracy of low grade gyroscopes. It is an object of the present invention to substantially overcome the above-identified disadvantages and drawbacks of the prior art.
U.S. Pat. No. 4,590,569, entitled xe2x80x9cNavigation System Including An Integrated Electronic Chart Displayxe2x80x9d, assigned to Navigation Sciences Inc. (Bethesda, Md.), describes a navigation system particularly adapted for ships making a passing within a harbor or the like. The system utilizes signal inputs from on-board vessel position determining equipment such as Loran or Decca apparatus and an on-board object detecting equipment such as a radar or sonar apparatus. The system further includes an on-board vessel position computer which operates in a differential Loran mode in response to observed Loran time differences, stored data from an initial calibration, and Loran grid offset data from an on-shore monitor system to compute a highly accurate current or present position fix in longitude and latitude whereupon the computer causes a predetermined electronic chart to be displayed in color on the screen of a cathode ray tube, being generated from a plurality of electronic charts stored in the form of digital files in memory. The selected chart, together with the present position of the ship, is displayed along with preselected alpha-numeric indicia of data relating to bearings, way points, ranges, xe2x80x9ctime to goxe2x80x9d, etc., also generated in accordance with the computed vessel position. Radar target returns of the local land mass and other stationary moving targets are additionally received by the ship""s radar. The radar image of the target echoes is next referenced to and superimposed on the electronic chart generated; however, the radar""s land mass echoes are suppressed in favor of the electronic chart land mass while displaying all other targets.
U.S. Pat. No. 5,194,872, entitled xe2x80x9cInertial Navigation System With Automatic Redundancy And Dynamic Compensation Of Gyroscope Drift Error,xe2x80x9d assigned to Charles Stark Draper Laboratory, Inc. (Cambridge, Mass.), describes an inertial navigation system with automatic redundancy and dynamically calculated gyroscopic drift compensation. The system utilizes three, two-degree of freedom gyroscopes arranged whereby any two of the gyroscopes form an orthogonal triad of measurement sensitive axes. The input axes of the three gyroscopes form three pairs of parallel input axes, each pair of parallel input axes corresponding to one axis of the orthogonal triad of axes. The three gyroscopes are operated in a plurality of pre-selected combinations of both clockwise and counter clockwise directions, thus changing the direction of the angular momentum vector by 180.degree. Parity equations are formed from each pair of gyroscope outputs whose measurement sensitive axes are parallel. The parity equations include combinations of gyroscope pairs that have been operated in both the clockwise and counterclockwise directions. Gyroscope drift estimates are then computed using the parity equations to provide individual gyroscope lumped drift corrections (self-calibration) to the inertial guidance and navigation system.
U.S. Pat. No. 5,527,003, entitled xe2x80x9cMethod For In-Field Updating Of The Gyro Thermal Calibration Of An Inertial Navigation Systemxe2x80x9d, assigned to Litton Systems, Inc. (Woodland Hills, Calif.), describes an in-field method for correcting the thermal bias error calibration of the gyros of a strapdown inertial navigation system. The method is begun after initial alignment while the aircraft remains parked with the inertial navigation system switched to navigation mode. Measurements are made of navigation system outputs and of gyro temperatures during this data collection period. A Kalman filter processes the navigation system outputs during this time to generate estimates of gyro bias error that are associated with the corresponding gyro temperature measurements. Heading error correcting is performed after the extended alignment data collection period as the aircraft taxis prior to takeoff. The gyro bias error-versus-temperature data acquired, along with the heading error corrections, are employed to recalibrate the existing thermal model of gyro bias error by means of an interpolation process that employs variance estimates as weighting factors.
U.S. Pat. No. 5,786,790, entitled xe2x80x9cOn-The-Fly Accuracy Enhancement For Civil GPS Receivers,xe2x80x9d assigned to Northrop Grumman Corporation (Los Angeles, Calif.), describes a method and means for enhancing the position accuracy of a civil or degraded accuracy GPS receiver by compensating for errors in its position solution with data derived from a military, or precise accuracy, GPS receiver. The civil GPS receiver may be disposed in a mobile expendable vehicle and the military receiver in a mobile launch vehicle. The compensating data is obtained by a comparison of the pseudorange measurements of the military GPS set and another civil GPS set disposed with it in the launch vehicle and attached to the same antenna. Two embodiments are disclosed involving variations of calibration, 1) an On-the-Fly Relative Navigation technique, applicable when the expendable receiver tracks the same satellites as the military and civil sets are tracking, wherein the position bias determined from the measurements of the two launch sets is transferred to the expendable receiver and used to offset its solution, and 2) an On-the-Fly Differential Navigation system, used when the expendable receiver is not tracking the same satellites as the launch sets, wherein the correction process is performed relative to the military set""s GPS position solution.
In one aspect, the invention comprises a method of calibrating acceleration data signals from a set of accelerometers, and angular rate data signals from a set of gyroscopes within a combined GPS/IGS. The method includes generating navigation data as a function of the acceleration data signals, the angular rate data signals, and prior navigation data. The method further includes combining the navigation data with GPS data via a Kalman filter, so as to produce corrected navigation data, navigation correction data, acceleration correction data and angular rate correction data. The method further includes modifying the acceleration data signals as a function of the acceleration correction data so as to calibrate the acceleration data signals, and modifying the angular rate data signals as a function of the angular rate correction data, so as to calibrate the angular data signals.
Another embodiment of the invention further includes receiving the set of acceleration data signals from a set of three mutually orthogonal accelerometers.
Another embodiment of the invention further includes receiving the set of angular rate data signals from a set of three mutually orthogonal gyroscopes.
In another embodiment of the invention, the acceleration correction data includes at least one acceleration bias correction factor. Modifying the acceleration data signals further includes adding the at least one acceleration bias correction factor to the acceleration data signals.
In another embodiment of the invention, the acceleration correction data includes three acceleration bias correction factors corresponding to three accelerometer data signals. The method further includes modifying the acceleration data signals further includes adding each of the three acceleration bias correction factors to the corresponding acceleration data signal.
Another embodiment of the invention further includes storing the at least one acceleration bias correction factor in a memory device, subsequently retrieving the at least one acceleration bias correction factor from the memory device, and adding the retrieved acceleration bias correction factor to the corresponding acceleration data signal.
In another embodiment of the invention, the angular rate correction data includes at least one gyroscope bias correction factor. Modifying the angular rate data signals further includes adding the at least one gyroscope bias correction factor to the angular rate data signals.
In another embodiment of the invention, the angular rate correction data includes three gyroscope bias correction factors corresponding to three angular rate data signals. Modifying the angular rate data signals further includes adding each of the three gyroscope bias correction factors to the corresponding angular rate data signal.
Another embodiment of the invention further includes storing the at least one angular rate bias correction factor in a memory device, subsequently retrieving the at least one angular rate bias correction factor from the memory device, and adding the retrieved angular rate bias correction factor to the corresponding angular rate data signal.
In another embodiment of the invention, the angular rate correction data includes at least one gyroscope scaling correction factor. Modifying the angular rate data signals further includes multiplying the at least one gyroscope scaling correction factor by the angular rate data signals.
In another embodiment of the invention, the angular rate correction data includes three gyroscope scaling correction factors corresponding to three angular rate data signals. Modifying the angular rate data signals further includes multiplying each of the three gyroscope scaling correction factors by the corresponding angular rate data signal.
Another embodiment of the invention further includes storing the at least one gyroscope scaling correction factor in a memory device, subsequently retrieving the at least one gyroscope scaling correction factor from the memory device, and multiplying the retrieved gyroscope scaling correction factor to the corresponding angular rate data signal.
In another embodiment of the invention, modifying the acceleration data signals further includes generating at least one acceleration bias correction factor. The at least one bias correction factor is a predetermined function of the acceleration correction data, the acceleration data signals, and parameters related to the accelerometers. The method further includes adding the at least one acceleration bias correction factor to the acceleration data signals.
In another embodiment of the invention, generating at least one acceleration bias correction factor further includes combining the acceleration correction data, the acceleration data signals, and one or more parameters related to the accelerometers as inputs to an algorithm. The algorithm may be implemented by a sequence of operations executed by a processor. Alternately, the algorithm may be implemented by a logic circuit.
In another embodiment of the invention, generating at least one acceleration bias correction factor further includes combining the acceleration correction data, the acceleration data signals, and one or more parameters related to the accelerometers as inputs to a look up table.
In another embodiment of the invention, modifying the angular rate data signals further includes generating at least one gyroscope bias correction factor. The at least one gyroscope bias correction factor is a predetermined function of the angular rate correction data, the angular data signals, and parameters related to the gyroscopes. The method further includes adding the at least one gyroscope bias correction factor to the angular rate data signals.
In another embodiment of the invention, generating at least one gyroscope bias correction factor further includes combining the angular rate correction data, the angular data signals, and parameters related to the gyroscopes as inputs to an algorithm. The algorithm may be implemented by a sequence of operations executed by a processor. Alternately, the algorithm may be implemented by a logic circuit.
In another embodiment of the invention, generating at least one gyroscope bias correction factor further includes combining the angular rate correction data, the angular data signals, and one or more parameters related to the gyroscopes as inputs to a look up table.
In another embodiment of the invention, modifying the angular rate data signals further includes generating at least one gyroscope scaling correction factor. The at least one gyroscope scaling factor is a predetermined function of the angular rate correction data, the angular data signals, and one or more parameters related to the gyroscopes, and multiplying the at least one gyroscope scaling correction factor by the angular rate data signals.
In another embodiment of the invention, generating at least one gyroscope scaling correction factor further includes combining the angular rate correction data, the angular data signals, and one or more parameters related to the gyroscopes as inputs to an algorithm. The algorithm may be implemented by a sequence of operations executed by a processor. Alternately, the algorithm may be implemented by a logic circuit.
In another embodiment of the invention, generating at least one gyroscope scaling correction factor further includes combining the angular rate correction data, the angular data signals, and one or more parameters related to the gyroscopes as inputs to a look up table.
In another embodiment of the invention, combining the navigation data with GPS data further includes receiving pseudo range rate data and pseudo rate data from the GPS. The method further includes combining the pseudo range rate data and pseudo rate data with the navigation data.
In another aspect, the invention comprises a system for calibrating acceleration data signals from a set of accelerometers, and angular rate data signals from a set of gyroscopes within a combined GPS/IGS. The system includes a navigation unit for generating navigation data as a function of the acceleration data signals, the angular rate data signals, and prior navigation data. The system further includes a Kalman filter for combining the navigation data with GPS data. The Kalman filter produces corrected navigation data, navigation correction data, acceleration correction data and angular rate correction data. The system further includes a compensator for modifying the acceleration data signals as a function of the acceleration correction data so as to calibrate the acceleration data signals. The compensator also modifies the angular rate data signals as a function of the angular rate correction data, so as to calibrate the angular data signals.
In another embodiment of the invention, the set of accelerometers is constructed and arranged so as to be mutually orthogonal.
In another embodiment of the invention, the set of gyroscopes is constructed and arranged so as to be mutually orthogonal.
In another embodiment of the invention, the acceleration correction data includes at least one acceleration bias correction factor, and the compensator includes an acceleration adder module for adding the at least one acceleration bias correction factor to the acceleration data signals.
In another embodiment of the invention, the acceleration correction data includes three acceleration bias correction factors corresponding to the three accelerometer data signals, and the compensator includes an acceleration adder module for adding each of the three acceleration bias correction factors to the corresponding acceleration data signals.
Another embodiment of the invention further includes a memory device for storing the at least one acceleration bias correction factor.
In another embodiment of the invention, the angular rate correction data includes at least one gyroscope bias correction factor, and the compensator includes a gyroscope adder module for adding the at least one gyroscope bias correction factor to the angular rate data signals.
In another embodiment of the invention, the angular rate correction data includes three gyroscope bias correction factors, and the compensator includes a gyroscope adder module for adding the three gyroscope bias correction factors to the corresponding angular rate data signals.
Another embodiment of the invention, further includes a memory device for storing the at least one gyroscope bias correction factor.
In another embodiment of the invention, the angular rate correction data includes at least one gyroscope scaling correction factor, and the compensator includes a gyroscope multiplier module for multiplying the at least one gyroscope scaling correction factor by the angular rate data signals.
In another embodiment of the invention, the angular rate correction data includes three gyroscope scaling correction factors, and the compensator includes a gyroscope multiplier module for multiplying the three gyroscope scaling correction factors by the corresponding angular rate data signals.
Another embodiment of the invention further includes a memory device for storing the at least one gyroscope scaling correction factor.
Another embodiment of the invention further includes an acceleration calculation module for generating at least one acceleration bias correction factor. The at least one acceleration bias correction factor is a predetermined function of the acceleration correction data, the acceleration data signals, and one or more parameters related to the accelerometers.
Another embodiment of the invention further including a gyroscope bias factor calculation module for generating at least one gyroscope bias correction factor. The gyroscope bias correction factor is a predetermined function of the angular rate correction data, the angular data signals, and one or more parameters related to the gyroscopes.
Another embodiment of the invention further includes a gyroscope scaling factor calculation module for generating at least one gyroscope scaling correction factor. The gyroscope scaling correction factor is a predetermined function of the angular rate correction data, the angular data signals, and one or more parameters related to the gyroscopes.
In another aspect, the invention comprises a system for calibrating acceleration data signals from a set of accelerometers, and angular rate data signals from a set of gyroscopes within a combined GPS/IGS. The system includes means for generating navigation data as a function of the acceleration data signals, the angular rate data signals, and prior navigation data. The system further includes means for combining the navigation data with GPS data, so as to produce corrected navigation data, navigation correction data, acceleration correction data and angular rate correction data. The system also includes means for modifying the acceleration data signals as a function of the acceleration correction data so as to calibrate the acceleration data signals, and means for modifying the angular rate data signals as a function of the angular rate correction data, so as to calibrate the angular data signals.